To conserve power, it is important to reduce power losses in transistors. In a metal oxide semiconductor field effect transistor (MOSFET) device, and in particular in the class of MOSFETs known as power MOSFETs, power losses can be reduced by reducing the device's on-resistance (Rdson).
Breakdown voltage provides an indication of a device's ability to withstand breakdown under reverse voltage conditions. Breakdown voltage is inversely related to Rdson, and so is adversely affected when Rdson is reduced. To address this issue, super junction (SJ) power MOSFETs, which include alternating p-type and n-type regions at the active regions of the device, were introduced. When the charges in the alternating p-type and n-type regions in a SJ power MOSFET are balanced (the charges in the p-type regions, Qp, are equal to the charges in the n-type regions, Qn), then breakdown voltage is at its peak value, thereby enabling the device to better withstand breakdown.
As Qn is increased relative to Qp, Rdson advantageously decreases. However, an n-channel SJ power MOSFET device operated with Qn greater than Qp will suffer from lower unclamped inductive switching (UIS) ruggedness, because the field peak at breakdown will occur closer to the base of the inherent parasitic bipolar transistor. Therefore, the device is generally operated with Qp greater than Qn. However, as Qp is increased relative to Qn, the breakdown voltage decreases and, consequently, the breakdown voltage will be less than its peak value for an n-channel SJ power MOSFET device operated in this manner.